A comparison has been made of the localization and ontogenetic development of acetyl-cholinesterase (AChE) and succinic dehydrogenase (SDH) in sections of fetal human spinal cord and brain stem using histochemical methods. Spinal cord sections of young fetuses (8–10 weeks in utero) showed low to moderate enzymatic activity whereas sections of older fetuses (12–18 weeks in utero) revealed high concentrations of AChE and SDH in the ventral and lateral horns. A similar increase in enzymatic activity during development was also observed in brain stem sections where AChE and SDH were mainly confined to cranial nerve nuclei. A comparison of our histochemical findings with investigations on the development of motor activity in the spinal cord suggests that there is a correlation between the increase in AChE activity and functional developm